1. Field of the Invention
The invention concerns data networks generally and more particularly concerns data networks that employ protocols belonging to the TCP/IP protocol suite and data networks that are asymmetric, that is, data networks in which there is more capacity to move data in one direction than there is in the reverse direction.
2. Description of the Prior Art
In the not-too-distant past, images could be processed and displayed only by large, special-purpose computer systems. Owners of lower-cost and less-powerful computers such as personal computers had to content themselves with character-based displays. The cost of memory has dropped so quickly and the power of microprocessors has increased so greatly in recent years, however, that modern personal computers are completely capable of processing and displaying images. Indeed, modern graphical user interfaces depend to a large extent on this capability.
Frustratingly enough for users of personal computers, the old problems with images have returned in another area, namely network computing. In network computing, the personal computer or work station is connected to a network and is able to use the network to fetch the data it is processing from remote locations. The most recent development in network computing is the Internet, a world-wide logical network which permits anyone who has access to the Internet to interactively fetch data including images from just about anywhere in the world. For example, using the Internet, it is possible to fetch pictures of the latest restoration projects in Florence, Italy from that city's home page on the World Wide Web.
The main drawback to interactively fetching data on the Internet is the length of time it takes to retrieve and display images. The problem is so serious that many people set up the program they use to access the Internet so that it does not fetch images. Doing this restricts the user to character data, but greatly decreases the time it takes to access information. The bottleneck in retrieving images from the Internet is not the personal computer, but rather the lack of capacity or bandwidth of the networks over which the images must be fetched. One part of the network where bandwidth is particularly restricted is the analog telephone line that connects most PC users to the Internet. It has been known for years that the bandwidth of the telephone system can be increased by replacing the analog system with a digital system, but all of the known techniques for doing this require extensive modification of the telephone system.
A great many homes do in fact have a high bandwidth connection, namely that provided by cable television. The problem with this connection is that it is one way. A PC may receive data via a home's CATV cable, but it cannot use the cable to send data. Again, ways of making the CATV system bidirectional have been known for years. For example, in the early 1980's, Scientific-Atlanta, Inc. introduced and marketed a product known as the Model 6404 Broadband Data Modem for use with bidirectional CATV systems. Scientific-Atlanta, Inc. has also recently filed U.S. patent applications Ser. Nos. 08/627,062, filed Apr. 3, 1996, 08/732,668, filed Oct. 16, 1996, and a continuation-in-part titled System and Method for Providing Statistics for Flexible Billing in a Cable Environment, Koperda, et al., filed Mar. 14, 1997 which describe bidirectional CATV systems. As with the telephone systems, the problem here is not the technology, but the fact that its introduction requires extensive modification of most existing CATV systems.
Given that many homes have a CATV cable and virtually all homes have an analog telephone line, systems have been proposed in which the CATV cable is used to send data from the Internet to the PC and the telephone line used to return data from the PC to the Internet. These systems take advantage of the fact that by far the most common pattern of interaction between users and networks is for the user to retrieve a large amount of data over the network, for example an image of a restored art work from Florence, examine the image, and then send a few keystrokes over the network. With this kind of interaction, far less bandwidth is needed in the channel that is used to return the keystrokes than in the channel that is used to fetch the image.
An example of such a system is the one disclosed in Moura et al., Asymmetric Hybrid Access System and Method U.S. Pat. No. 5,586,121, issued Dec. 17, 1996, and in Moura et al., Remote Link Adapter for use in TV Broadcast Data Transmission System, U.S. Pat. No. 5,347,304, issued Sep. 13, 1994. In this system, the head end of a cable system has high bandwidth access to the Internet or to other networks and access via CATV cables and the telephone system to households or businesses with PCs. Data received from these networks is sent to PCs connected to the cable system's cables and responses from the PCs are collected via the telephone system and sent to the network. In the home or business, the PC is connected either directly or via a local area network to a device which includes both a radio frequency modem and a standard analog telephone modem. The radio frequency modem is connected to the CATV cable. It receives and decodes the data sent on the CATV cable and provides it to the PC. The telephone modem is connected to a standard telephone line. It receives data from the PC and sends it to the CATV head end, which in turn forwards it to the Internet or other networks.
While systems such as the one disclosed in the Moura references do provide a solution to the bandwidth problem, they have a number of deficiencies, particularly when used in the context of the Internet. Among the deficiencies are the following:
The system of Moura wastes Internet Protocol (IP) addresses for the computers attached to the modem. IP addresses are in short supply. In the system of Moura, however, IP addresses are statically assigned to the PCs and are consequently not available for reuse when a PC is idle or not engaged in an activity which involves network access.
From the point of view of the Internet, the system of Moura is a link level system, that is, the components of the system of Moura do not themselves have IP addresses and cannot themselves execute IP protocols. In particular, IP routing is not used within the system of Moura. One difficulty arising from this situation is that IP routing is centralized in the IP router that connects the head end to the Internet; another is that the modem in the system of Moura cannot function as an IP router.
In Moura, the telephone connection to the modem is used solely to transfer data from the PC and modem to the head end. All data received by the PC and modem is sent via the CATV cable. Consequently, when the CATV system fails, the PC is left without a connection by which it can receive data. This situation is made even less desirable by the fact that CATV systems are far more likely to fail than the telephone system.
The CATV channel to which the modem of Moura responds is statically assigned to a given modem, thereby rendering the channel unavailable for use by other modems when the PC connected to the given modem is idle or is not engaged in an activity which involves network access.
It is an object of the system disclosed herein to overcome the preceding and other deficiencies of systems like that of Moura.